In an effort to ensure the intended performance of a communication system, namely, the ability of a receiving station to successfully recover the information that was originally transmitted from a transmission station, the system is customarily provided with error or distortion monitoring equipment from which an indication of errors in the received signals may be obtained.
Examples of error detection circuits that have been proposed include those employing threshold detectors which monitor the variation of the demodulated signal above or below a prescribed threshold. For instance, adjustment of the threshold value may be required where the signal level undergoes long-term fluctuations that are not due to interference. When employed in a phase modulation system, operation of the threshold detector depends primarily on the use of automatic gain control circuitry to adjust the signal relative to a manually set threshold. An obvious drawback to manually setting the signal level relative to the threshold value is that changes in the channel or link over which the communication is conveyed directly impact on the manual setting.
To overcome the shortcomings of manual adjustment, there have been proposed threshold detectors that are adjusted automatically. One such circuit which is described in U.S. Pat. No. 3,638,183 to Progler et al. adjusts the detector in dependence upon the peak value of a received signal that persists for a prescribed period of time. This circuit employs a threshold circuit including a network, the transfer function of which is less than unity, connected in parallel with the series connection of a peak value rectifier and a memory delay. The memory delay transmits only those level fluctuations which persist in excess of a prescribed time interval. The memory delay and the transfer network are connected to a comparator which generates an output signal when the values of the outputs of the memory delay and the transfer network differ by more than a fixed amount.
Now, although the Progler et al approach seeks to provide automatic threshold adjustment, it suffers from the drawback that it averages gain over a considerable period of time, so that it is not readily employable in any type of communication system.
In a TDMA (Time Division Multiple Access) satellite communication system, the signal demodulator receives a large number of bursts of information, and usually, the bursts of information from a particular transmitting station occur once per frame. The conventional averaging circuits tend to average the received gain over many bursts, and therefore, cannot individually control the amplitude of a burst from a specific station. Yet, in order to provide an accurate estimate of error for a particular link, it is an a priori requirement that only the bursts from an individual station be measured. In order to measure the bit amplitudes accurately, the incoming signal waveform must be measured at the same sample points that are used for making bit decisions. These requirements of such a communication system further demonstrate the shortcomings of a threshold adjustment circuit as described in the above referenced Progler patent, wherein the threshold value is established in accordance with changes in the level of the received signal that persists for a relatively long period of time. The measured input signal may be adversely affected by waveform distortion, which waveform distortion is not reflected in the threshold value with which it is compared, since the threshold depends upon changes in the level of the received signal which persists over a long time period. At the data rates involved in satellite communications, the waveform distortion would not necessarily persist long enough for the Progler system to properly adjust its threshold value, particularly where the distortion is generated by switching operations in the signal recovery circuitry.